The present invention relates to a positive active material for nonaqueous electrolyte secondary batteries useful as a power source in portable electronic or communications equipment, electric cars, and the like, and to a nonaqueous electrolyte secondary battery using the positive active material.
Lithium ion secondary batteries, which are one class of nonaqueous electrode secondary batteries, have advantages including a high voltage, a high energy density, and a low self discharge, and they have become indispensable as a power source for portable electronic or communications equipment such as mobile phones, laptop personal computers, camcorders, and the like.
Lithium ion secondary batteries which are currently in practical use are 4V-grade batteries in which a carbon material such as graphite is used for the negative electrode, LiCoO2 (lithium cobaltate) is used as an active material for the positive electrode, and a nonaqueous solution of a lithium salt in an organic solvent is used as the electrolytic solution. When these batteries are charged, the following reaction occurs:
Charge reaction: LiCoO2+nC6xe2x86x92Li(1-n)CoO2+nLiC6.
A high voltage of at least 4.8 V is required for full (100%) charge of the batteries. However, such a high voltage may cause decomposition of the electrolytic solution and adversely affect the reversibility of charge and discharge reactions, resulting in a loss of the cycle life of the secondary batteries. Therefore, in practice, the maximum voltage is limited to 4.1-4.2 V. Thus, the positive active material is utilized in a stable region where the value of xe2x80x9cnxe2x80x9d in the above reaction is around 0.5, so the positive active material as charged can be expressed approximately as Li0.5CoO2.
As the performance of portable electronic or communications equipment is increased, it is increasingly required for secondary batteries to have a high energy density with a small size and a light weight. On the other hand, in the field of large-sized secondary batteries for use in electric cars which are under development with a view of maintaining the global environment, there is a demand for secondary batteries which not only have a high energy density but also are safe.
In addition, the costs of secondary batteries are important, particularly for large-sized secondary batteries. The use of an expensive cobalt compound whose resources are limited as a positive active material necessarily adds to the cost of the above-described practical lithium ion secondary batteries. The high cost of lithium ion secondary batteries is a major cause when they are precluded from being mounted in electric cars.
It is well known that LiNiO2 (lithium nickelate) can also be used as a positive active material for lithium ion secondary batteries. Like LiCoO2, LiNiO2 has a layered, hexagonal crystal structure and allows lithium (Li) ions to be intercalated and deintercalated between layers of the crystal structure. The charge reaction occurring when LiNiO2 is used as a positive active material is basically the same as the above-described charge reaction for LiCoO2. However, LiNiO2 can be charged in a stable manner until the value of xe2x80x9cnxe2x80x9d in the above charge reaction formula reaches around 0.7. Thus, in this case, the positive active material as charged can be expressed approximately as Li0.3NiO2, thereby constituting a positive electrode of a higher capacity.
Compared to LiCoO2, LiNiO2 has the advantages of being less expensive and being capable of making a secondary battery of higher capacity. However, LiNiO2 has the problem that its crystal structure tends to be broken during charging and discharging, thereby adversely affecting the cycle properties of the secondary battery. In addition, in a secondary battery having a positive active material of LiNiO2, an exothermic decomposition reaction may occur when the charged positive active material is exposed to a high temperature in the presence of the electrolytic solution, whereby the active material is converted into a compound approximately expressed as Li2Ni8O10 and oxygen is liberated. The liberated, active oxygen may react with the electrolytic solution or another component, or serve as combustion-promoting oxygen. As a result, there may be a risk of igniting the battery itself in some cases. Thus, a secondary battery using LiNiO2 as a positive active material has poor thermal stability, and this positive active material could not be used in practical batteries.
As an attempt to improve the cycle properties of a secondary battery using LiNiO2 as a positive active material, stabilizing the crystal structure of this compound by replacing part of Ni by another element such as Co was investigated, as described in Solid State Ionics, 90, 83 (1996). This approach makes it possible to considerably improve the cycle properties.
On the other hand, with respect to the thermal stability of a secondary battery using LiNiO2 as a positive active material, it was reported in the 40th Symposium on Batteries in Japan (1999), Presentation Number 1C12 that when part of Ni is replaced by Co+Mn, the thermal stability can be improved as the amount of replacing Co increases or the Ni content decreases with a certain amount of replacing Mn. With this approach, however, it is difficult to improve the thermal stability to the same level as that of LiCoO2, although an initial capacity surpassing that of LiCoO2 can be obtained.
A LiCoO2-based positive active material containing at least one element selected from Cu, Zn, Nb, Mo, and W is described in JP-A 06-283174. Although it is explained therein that the positive active material has a high capacity and good cycle properties, the cycle properties are measured with only ten cycles and do not yet reach a level sufficient for practical use.
As discussed above, with LiNiO2-based materials which are positive active materials less expensive than LiCoO2, although it is possible to attain a high capacity surpassing that attainable with LiCoO2, the thermal stability of the positive active materials in their charged state is poor, and it is difficult to improve the thermal stability to the same level as that of LiCoO2. Thus, none of these materials have been improved in both initial capacity and thermal stability.
It is an object of the present invention to develop a LiNiO2-based positive active material for use in nonaqueous electrolyte secondary batteries which has an initial capacity higher than that of LiCoO2 and which is improved in thermal stability in a charged state at least to the same level as LiCoO2, thereby making it possible to provide nonaqueous electrolyte secondary batteries which are less expensive and have better performance than the current practical lithium ion secondary batteries.
The present inventors found that the thermal stability of a LiNiO2-based positive active material can be improved at least to the same level as that of LiCoO2 by replacing part of the Ni in LiNiO2 by Co and Mn and further by one or both of W and Mo. Although the invention is not intended to be bound by a specific theory, it is presumed that the improvement in thermal stability results from suppression of oxygen liberation which is caused by decomposition of the positive active material during charging and also from shifting the decomposition temperature to a higher temperature.
The present invention is a positive active material for use in nonaqueous electrolyte secondary batteries, characterized in that it is comprised of a lithium compound oxide (compound oxide of lithium) of the formula:
LiaNibCocMndMeO2xe2x80x83xe2x80x83(1)
where M stands for one or two of W and Mo,
0.90xe2x89xa6axe2x89xa61.15, 0 less than b less than 0.99, 0 less than cxe2x89xa60.5, 0 less than dxe2x89xa60.5, 0 less than c+dxe2x89xa60.9, 0.01xe2x89xa6exe2x89xa60.1, and b+c+d+e=1,
and in that the lithium compound oxide gives an X-ray diffraction pattern including a diffraction peak or peaks assigned to a compound oxide of Li and W and/or a compound oxide of Li and Mo, in addition to main diffraction peaks assigned to a hexagonal crystal structure.
The present invention also relates to a nonaqueous electrolyte secondary battery comprising a negative electrode comprised of lithium metal or a substance capable of absorbing and desorbing Li or Li ions and a positive electrode comprised of the above-described positive active material.
The positive active material for nonaqueous electrolyte secondary batteries having a composition shown by the above formula (1) according to the present invention has significantly improved thermal stability in a charged state. As can be seen from the DSC (differential scanning calorimeter) diagrams given in the examples described below, the improved thermal stability is not only superior to that of LiNiO2, but is also superior to that of conventional thermally-stabilized LiNiO2-based positive active materials in which part of Ni is replaced by Co and Mn, and is even superior to that of LiCoO2 which is inherently thermally stable. Thus, addition of an extremely small amount of W and/or Mo provides a significant improvement in thermal stability without a significant decrease in initial capacity. The effect of W and/or Mo on improvement in thermal stability is not known and has been first found by the present inventors. As a result, the present invention makes it possible to manufacture nonaqueous electrolyte secondary batteries having both good thermal stability and good initial capacity.